How Do Radios Produce Sound?

Radio began the communication revolution. Unlike telegraphy, which linked two specially trained operators, or telephony, which linked only two speakers, radio had the potential to link millions to information, entertainment and public affairs. Broadcasting consists of changing electromagnetic waves with frequencies below those of visible light in two ways: modulation and amplification. Modulation allows the manipulation of waves so that a receiver "hears" different frequencies in succession, which register as different sounds to the human ear. Amplification increases the range of sound that can be transmitted. The first radio microphones were large electromagnets that transformed sound waves into electromagnetic waves. These signals were received by "coherers"---detectors consisting of metal filings in a primitive vacuum tube that processed the waves into audible sounds as they were moved by the electromagnetic "signal." The problem with early radio was that the signal didn't travel very far so broadcasts only reached local listeners who had the proper receivers and coding training. Basically, it was a telegraph that was wireless.

After the invention of the vacuum tube, broadcasters could increase the reach of radio signals. Amplitude modulation (or AM) was used to code signals that could travel 100 miles and today, "clear channel" AM stations can reach across the country. The receivers for these signals took the electromagnetic signal from the antenna and conducted it through a series of circuits and vacuum tubes that worked like sophisticated, powerful coherers to convert, or "demodulate," the electromagnetic waves back into sound waves using amplitude modulation. Volume was controlled by variable resistance; turning the volume dial allowed more current to carry audio waves to the radio's speaker, increasing perceived volume. Experiments began in the 1930's to use frequency modulation (FM) to carry waves and new "tuners" had to be developed to demodulate the signals. FM made it possible to use broadcast at lower frequencies and eliminate much of the background static that was present in the higher AM range. Throughout the first half of the century, vacuum tubes and wire circuits provided the basic technology used to produce sound.

In the latter half of the 20th century, communications technology changed radio as well as making possible a wide range of communication devices, from television to baby pagers. First, miniaturized power sources for receivers (batteries) made possible a demodulator that did not have to be connected to an electrical supply, then transistors replaced bulky vacuum tubes and printed circuits replaced wires. Receivers that converted radio waves into visible patterns established the foundation for television. Today, radio uses the same applications of the theory of electromagnetism developed by Nikola Tesla in 1893 in cell phones and police radios. New technology using software and digital receivers has made web and satellite broadcasting possible, extending the range of broadcasting beyond that imagined by its early pioneers.